There’s not much that will create an argument more than asking “What is the best firing stance and grip?” in a forum of AR shooters. While everyone has an opinion and many feel their’s is the only one that counts I’ll give you my quick answer – “The one that you are comfortable with and allows rounds to be sent down range – accurately.”
Several years ago I heard of this method competition shooters were using to control recoil and shoot very accurately – called the “C-Clamp”. I tried and hated it. Man – it felt so uncomfortable. I preferred to hold the magazine well however I just could not control recoil well and get multiple rounds on target quickly. I had signed up for a defensive carbine and pistol course from Wolf-Fire Firearms Training and decided to try the C-Clamp again. I used it throughout the class and it really grew on me. Now, this is the primary method I use for shooting the AR.
The video below gives some perspective on the “thumb over bore” and explains my position on it well. I agree with this guy 100%.